Mazzello was born in Rhinebeck, New York, and raised in Hyde Park, New York, the son of Virginia (née Strong) and Italian American Joseph Mazzello, Jr., who owned a dance studio. He has one older sister, Mary, and a younger brother, John, who have both appeared in films. His father is of three-quarters Italian and one-quarter German Jewish ancestry, while his mother is of Irish and English descent. Mazzello went to the Catholic school Our Lady of Lourdes. He is an alumnus of the University of Southern California, entering the USC School of Cinematic Arts in 2001 following a recommendation letter from director Steven Spielberg. Mazzello paid for school with his salary from a small appearance in The Lost World: Jurassic Park (1997), something the actor jokingly referred to as his graduation present from Spielberg.